1. Field of the Invention
This invention relates to apparatus for scanning a modulated light-beam across a photosensitive surface, and especially apparatus commonly known as platesetters, in which an image is scanned directly onto the photosensitive surface of a printing plate.
2. Description of the Background
The plates used in platesetters are generally made of aluminium sheet and are supplied in different sizes to suit different size printing jobs. In use, each plate is mounted on a scanning bed of the platesetter, which is typically a fixed concave bed to suit the curved focal plane of a particular rotary mirror, light-beam scanning system. The aluminium plates are thin enough to be flexible so as to conform to the concave shape of the bed, and are held in position by mechanical fastenings and/or pneumatic suction means.
The quality of the print image produced is principally dependent upon the accuracy of the light-beam scanning system, and this is why curved bed platesetters have been preferred. The productivity of a platesetter is dependent upon the speed of operation of the scanning system. However, the time taken in loading and removing plates from the bed also has a major effect on productivity and also effects quality adversely if the plates are not secured adequately, especially at the edges where their natural resilience can cause them to lift away from the bed.